29.1613GPPInterworking between the Public Land Mobile Network (PLMN) supporting packet based services with Wireless Local Area Network (WLAN) access and Packet data Networks (PDN)TS
In current LAN environments the most commonly used configuration protocol is DHCP (Dynamic Host Configuration Protocol, RFC 2131 ) and DHCPv6 (Dynamic Host Configuration Protocol for IPv6, RFC 3315 ). It provides a mechanism for passing a large set of configuration parameters to hosts connected to a TCP/IP network (IP address, sub‑net mask, domain name, MTU, etc.) in an automatic manner. Moreover DHCP may assign IP addresses to clients for a finite lease time, allowing for sequential reassignment of addresses to different users.
The lease time is chosen by the administrator of the DHCP server (in the external network), and is therefore out of the scope of the present document.
The Packet Domain offers the end user the possibility to run DHCP end-to-end the same way as he does when connected directly to a LAN (e.g. an enterprise Intranet). No modifications should be required in common implementations of DHCP clients and servers. However a Packet Domain-specific DHCP relay agent, refer to RFC 2131  and RFC 3315 ,is needed in the PDG so as to allow correct routing of DHCP requests and replies between the WLAN UE and the DHCP servers.
9.2 Address allocation by the Intranet or ISP
Address allocation schemes using DHCP are described in subclause "Access to Internet, Intranet or ISP through Packet Domain", with details for IPv4 in RFC 2131  and for IPv6 in RFC 3315 .
9.3 Other configuration by the Intranet or ISP (IPv6 only)
When using IPv6, in some situations the WLAN UE may need additional configuration information from the Intranet or ISP besides the IP address. It may for example be IMS related configuration options (see 3GPP TS 24.229 ). If the UE is DHCP capable and the IPv6 address has been allocated using Stateless Address Autoconfiguration, the UE may use a procedure as in the example in figure 5 to configure additional external network protocol parameters, or other parameters that apply to the Intranet or ISP. The PDG shall in this case indicate to the UE that there is additional configuration information to retrieve by setting the O-flag in the Router Advertisements. This shall be configured per W-APN in the PDG.
The following bullets describe an example of a signal flow, where the UE directs an Information-Request to the All_DHCP_Relay_Agents_and_Servers multicast address. The UE may also direct the message to a specific server instead of all servers. For a detailed description of the DHCPv6 messages refer to the DHCPv6 RFC 3315 . The sequence is depicted in figure 5.
1) A Router Advertisement with the O-flag set is sent from the PDG to the UE to indicate to it to retrieve other configuration information.
2) The UE sends an INFORMATION-REQUEST message with the IP destination address set to the All_DHCP_Relay_Agents_and_Servers multicast address defined in the DHCPv6 RFC 3315 . The source address shall be the link-local address of the UE. The DHCP relay agent in the PDG shall forward the message.
3) DHCP servers receiving the forwarded INFORMATION-REQUEST message reply by sending a RELAY‑REPLY message, with the "Relay Message" option including a REPLY message with the requested configuration parameters.
The UE chooses one of the possibly several REPLY messages and extracts the configuration information.
Figure 5: DHCPv6 other configuration signal flow